A junction diode is a fundamental semiconductor device which presents low impedance for polarization above a threshold voltage in forward polarization (i.e. positive voltage applied to p-type semiconductor and negative voltage applied to n-type semiconductor), typically 0.6 V for silicon p-n diodes, and a much higher impedance in reverse polarization. Essentially, an ideal diode is an electrical conductor in one direction of current flow and an insulator in the opposite direction.
A photodetector diode is a diode which is conduction sensitive to light absorption. Photodetector diodes are constructed for response to light of specific ranges of wavelengths, including but not limited to visible, ultraviolet, and infrared. For example, x-ray detector and gamma-ray detector diodes conduct upon detecting other high-energy photons.
In solid-state physics, a band gap, also called an energy gap or bandgap, is an energy range in a solid where no electron states can exist. In graphs of the electronic band structure of solids, the band gap generally refers to the energy difference (in electron volts “eV”) between the top of the valence band and the bottom of the conduction band in insulators and semiconductors. Bandgap is the minimum energy required to promote a valence electron bound to an atom in a solid to become a conduction electron, which is free to move within the crystal lattice and serve as a charge carrier to conduct electric current.